I like YA author Janet Lee Carey's method of fleshing out realistic characters:
JLC: The character has a life before he or she steps into the book. I put a lot of thought into backstory looking at where the character came from and who influenced them the most growing up. I also look for what drives the character — what they long for, hunger for. One way to find this is to have the character finish this sentence, “More than anything, I want _______________.” Make what the character most wants seem completely out of reach and you’ve got the beginning of a plot. The character has to take the plot personally for the story to have momentum.
From CZX interview Mar. 16, 2010